Today, my ex got dumped by his girlfriend. Seeing my opportunity I sent him a song I wrote for him about how much I still love him. He sent it on to the girl who had just dumped him claiming he had written it. They are back together. FML

I agree, you should tell her, and you do both deserve better.
Buuuuuuuut, I'm thinking the OP is the one who'll get in trouble there, for moving in so quickly on this bloke. Either way, still tell her. The dude's a tool, and she should know the extent of it :P

Don't tell his girlfriend. How about you realize what you already should have: He's an ex for a reason. YDI for being so freaking desperate. As they say, there are other fish in the sea. Or, He's just not that into you.

Nah, genius, I think she SHOULD tell the girlfriend, after all it's HER song. She does need to know he's a twit, before she gets too deep.
But I agree that moving in like that was pretty desperate; yeah, exes are usually exes for a reason.
So FYL and YDI, OP.

I don't think so. She needs to move on with her life - his girlfriend knowing the truth is none of her concern. Being vindictive is not an attractive trait, certainly not something anyone looks for in a potential partner - taking the high road detaches her from the situation, and that's what she needs.
OP: the guy doesn't care about you, he cares about that other girl. He doesn't mind taking something from you, he doesn't mind hurting you. You expected differently from someone you love, but now you know he's not right for you. You also know now that you can write good songs. Move on and write the next one for the right person - good luck.

@102: She SHOULD move on, but at the same time she should tell the girlfriend about it because it's plagiarism. It's like saying you should just let it go if your best friend decided to hand in your painting as his own to a teacher or something. She doesn't have to be bitter about it to still do that.
FYL for having your song stolen. At least now you should realize how much of a douche he is and how you SHOULDN'T be pining for him anymore.

I think that's not a good idea. The difference between him handing it in to a teacher, and him claiming it was his to his girlfriend, is that the teacher might matter in the OP's life - her ex' lover should not. If she keeps talking to these people she keeps them in her life. The right time to cut them out of her life is now - not after doing that one last thing. She put it off long enough already.
In all likelihood the new gf wouldn't believe her anyway. What should she do then? Argue with her? Try to prove it to her somehow? What if new gf she believes her - don't you think the boyfriend wouldn't call her and whine about it? That's not a good way to get him out of the OPs life. What if the new gf believes her, forgives the boyfriend and calls the OP a bitch who tries to destroy their relationship?
There are thousands of guys telling their girlfriends lies, right this minute. That's none of her concern, just like these two are no longer her concern. She needs to get out now.

I don't know, st0815, some people just don't appreciate other people taking credit for their work. I'd tell the new girlfriend about the origins of the song in a mature, calm manner just so she'd know that her new boyfriend is ignorant about plagiarism; then, if she chooses not to believe it, let her suffer for it with "I told ya so!" echoing in the back of her mind.

Do what #25 told you OP. If you show her the song your EX's gf will probably either:
You're right, or,
Notice the song lyrics were correct and suppose your EX sang the same song to you and her, which acting as if he's cheating on her.

haha he obviously didn't want to be with you and what did you expect to happen, him to come rushing towards you, you both kiss romantically and then marry and have kids together? hahaha YDI for being Naive buy yh FYL lol

TELL HER GODDAMMIT!
Although don't try and hook back up with him as:
A. It will look like you were just trying to get yher to break up with him so you could have him
B. He's an ass and uninterested in you. And an ass.